Two BIG Thistle improvements

I've made two fairly significant improvements to Thistle, each of which is pretty exciting in its own right:

Permalinks will now always load almost instantly, regardless of how many posts are in your blog. Obviously,
if the post you're loading is big or has graphics or whatever, it will take a bit longer. The point is that
the time needed for Thistle to prepare to load the post has been slashed. I'm seeing times of less than 1
second, in a test blog of about 100 medium-sized (4-10k) entries.

Thistle will now drastically out-perform PHPosxom in loading permalinks; I'll be giving Robert the code to integrate into PHPosxom
soon. And I have several other optimisations planned.

You can now create hidden posts. Hidden posts are posts which show up as normal when you directly visit their permalink,
but are not listed in the main blog. This is achieved by putting a post in one of your $ignore_folders.

Naturally, you can still do all the fancy category-specific overriding for your "hidden posts" category, giving you full
control over their appearance. You could use this feature for almost anything, from just previewing your posts before actually
publishing them, right up to implementing some kind of "drafts" system (Robert, remember our conversation about this today? ;)).